I. Field
The disclosed embodiments relate to wireless communications. More particularly, the disclosed embodiments relate to a novel and improved method and apparatus for controlling transmission energy in a wireless communication system.
II. Background
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation technique of CDMA has significant advantages over these modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, which is assigned to the assignee of the present invention and fully incorporated herein by reference. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, which is assigned to the assignee of the present invention and fully incorporated herein by reference.
CDMA by its inherent nature of being a wideband signal offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled “METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM”, and U.S. Pat. No. 5,109,390 entitled “DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM”, both assigned to the assignee of the present invention and incorporated by reference herein.
In a communication system, such as CDMA, that provides data using a Quaternary Phase Shift Keying (QPSK) modulation format, information regarding the transmitted data signal can be obtained by taking the cross product of the I and Q components of the QPSK signal with the estimate of the communications channel. By knowing the relative phases of the two components, one can determine roughly the velocity of the mobile station in relation to the base station. A description of a circuit for determining the cross product of the I and Q components with the channel estimate in a QPSK modulation communication system is disclosed in U.S. Pat. No. 5,506,865, entitled “PILOT CARRIER DOT PRODUCT CIRCUIT”, assigned to the assignee of the present invention, the disclosure of which is incorporated by reference herein.
There has been an increasing demand for wireless communications systems to be able to transmit digital information at high rates. One method for sending high rate digital data from a central base station to a subscriber unit is to allow the base station to send the data using spread spectrum techniques of CDMA. One proposed method allows the remote station to transmit its information using a small set of orthogonal channels, this method is described in detail in copending U.S. Pat. Ser. No. 08/886,604, entitled “HIGH DATA RATE CDMA WIRELESS COMMUNICATION SYSTEM”, assigned to the assignee of the present invention and incorporated by reference herein.
Power control is a necessary component of wireless communication systems. A conventional method of power control of a mobile station, or reverse link power control, in a communication system is to monitor the power of the received signal from the mobile station at a base station. The base station in response to the monitored power level transmits power control bits to the mobile station at regular intervals. A method and apparatus for controlling transmission power in this fashion is disclosed in U.S. Pat. No. 5,056,109, entitled “METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM”, which is assigned to the assignee of the present invention and incorporated by reference herein.
However, conventional methods of power control may not apply to the forward link in high data rate systems. Conventional methods of power control typically use known properties of pilot signals to estimate channel characteristics. In the forward link of high data rate systems, these known properties may not be present. The forward link pilot signal in a high data rate system may not be power controlled for a single subscriber unit, and also, may not be transmitted on a dedicated pilot channel.
There is a need in high data rate wireless communication systems for an efficient method of power control of a base station, or forward link power control. There is also a need in high data rate wireless communication systems to monitor the power of the received signal from the base station at a subscriber unit, and in response to the monitored power level, transmit power control bits from the subscriber unit on the reverse link to the base station at regular intervals.